The developer’s website has a detailed FAQ section on that. In short, though Valve doesn’t seem to have explained or told the developer what needs to be removed or censored, the developer thinks it’s to do with an early pre-release scene where a man brought his teen daughter to the horse farm, where she sat on the shoulders of a naked woman representing one of the farmer’s horses, who in the game are used and abused like their real-world equivalents. Though the developer has subsequently replaced the girl with a 20 year old and pixelated the nudity, Valve and Epic still didn’t find the game acceptable.